We propose to produce human monoclonal antibodies to cytomegalovirus (CMV) as a potential unlimited and safe source of antigen-specific antibodies and to analyze their viral antigen specificity and functional effects on the immune response to virus in normal and immunocompromised individuals. While impaired cellular immunity is thought to be the major contributing factor in immunocompromised patients, administration of human plasma with high anti-CMB titers modifies with severity or eliminate CMV infections. Among bone marrow and organ transplant recipients, reactive or acquired herpesvirus infections frequently occur after transplantation, with blood or blood component transfusions as a significant vector in virus acquisition. (CMV) is the herpesvirus most commonly associated with significant morbidity and mortality. Normal individuals, when challenged with a viral infection, generate cytotoxic lymphoid and non-lymphoid killer cells, as well as viral specific antibody which can neutralize virus directly, as well as provide a "bridge" between lymphoid killer cells and infected target cells in antibody-dependent cellular cytotoxicity. A human-mouse derived myeloma line, shown to fuse efficiently to human B cells and produce stable human immunoglobulin secreting hybridomas, will be fused to B cells of anti-CMV seropositive patients with demonstrable in vitro secretion of anti-CMV antibodies to produce a panel of human monoclonal antibody against CMV. Using this system, several human monoclonal antibodies against CMV have already been produced. The effects of these antibodies and others to be produced on antigen specific immune function will be explored by adding antibody to in vitro assays. Finally, the target molecules of these antibodies will be radiolabeled in virus infectioned cells and analyzed structurally by polyacrylamide gel electrophoresis. These studies should provide new insights into the basic mechanisms underlying the immune response to viral infections. These antibodies should also be useful in the diagnosis and treatment of CMV infections.